Add weight to all vertex groups on an object?
select the object  set the addition value and Run this script :

import bpy

addition = 0.1

obj = bpy.context.active_object
obj_data = obj.data
bpy.ops.object.mode_set(mode = 'OBJECT')
for vert in obj_data.vertices:
    for g in obj_data.vertices[vert.index].groups:
            g.weight  = g.weight +addition